1. Field of the Invention
The present invention relates to a method for fabricating an isolation for a semiconductor device, and more particularly to a method for fabricating a local oxidation of silicon (LOCOS) isolation.
2. Description of the prior art
To improve the bird's beak (encroachment effect) at the edge of a LOCOS isolation, a polysilicon spacer serving as a buffer layer is widely used in the LOCOS isolation process.
Referring to FIGS. 1A through 1E, the cross-sectional side views of a conventional method for fabricating a LOCOS isolation are depicted in sequence.
Referring now to FIG. 1A, a cross-sectional view of the starting step is schematically shown. In FIG. 1A, a first pad oxide layer 12 and a silicon nitride layer 14 are sequentially formed on the active region of a silicon substrate 10. Then, using silicon nitride layer 14 as an etching mask, the silicon substrate 10 is etched to form a recess 15 by anisotropic etching.
Next, as shown in FIG. 1B, a second pad oxide 16 is formed on the surface of the recess 15 by thermal oxidation, which preferably uses a gas containing O.sub.2 as the reactive gas.
Now as shown in FIG. 1C, a polysilicon spacer 18, which extends the recess 15, is formed on the side walls of the silicon nitride layer 14. The polysilicon spacer 18 can be formed by the steps of depositing, and etching back a polysilicon layer. A thin silicon nitride layer 20 is deposited on the surface of the polysilicon spacer 18 by selective chemical vapor deposition.
Referring now to FIGS. 1C and 1D, a local oxide 22 (isolation), which has a deeper concave portion 24, is grown above the recess 15 by thermal oxidation.
Next, referring to FIG. 1E, the silicon nitride layer 14 is removed via wet etching to leave local oxide 22a.
However, the LOCOS isolation process suffers from problems, for example the deeper concave portion 24 can cause an uneven surface. Moreover, the thin silicon nitride layer 20 formed on the surface of the polysilicon spacer 18 limits the oxidation of polysilicon spacer 18. Therefore, a portion of polysilicon spacer 18 does not react into local oxide 22 during the thermal oxidation step. Also, the conventional method described above may cause the encroachment to be too large.